Faulty LNB

It might be that your dish is out of alignment.  Do you get error messages when you try to tune one of those channels?

If it's an LNB problem, you can't just replace one of the LNBs, they share electronics. You have to get a triple LNB assembly, for the Phase III dish , like this one https://www.solidsignal.com/directv-triple-lnb-for-phase-iii-satellite-dish-lnb-salnb3

You might get one on ebay for less but make sure it's for the phase III dish, NOT for the slimline.

Or you can call DirecTV for a service visit, for $100 the tech will align your dish, and replace anything that needs replacing. The tech might even replace the whole dish because the phase III is very old.

What are the models of each box you have?

Do they each have the problem?

What happens on those channels when you try to watch them? (symptoms, errors, etc.)

If you have a Phase III dish then that may be part of the problem. DirecTV has been shutting down their MPEG-2/SD-only feeds as they are very outdated.

Instead of a $99 service call, request a MPEG Swap which should get you MPEG-4 boxes and a new dish installed at no cost. Just make sure you do not say "upgrade" or you get a 24 month agreement and adding HD service to your bill. Whether you do required MPEG swap or optional upgrade, read order confirmation to verify all is as expected.

Sorry, I was stupid!!. Mea culpa. Yes, there is no point in replacing the LNB assembly because  two of the LNBs currently do nothing, and the third only gets the sd signals from the 101 satellite. As Juniper says, you MUST get the free MPEG swap (receiver and dish) because otherwise soon you will not be able to get Directv at all.